Given Steam's upcoming community update with extra emphasis on user generated content, I'd say a logical next update for Team Fortress 2 would be to increase mod-friendliness. Some might argue Valve's Source games are already very mod-friendly, but look at what they did with the Portal 2 level editor. Not until that thing was released did the number of custom maps available explode. I imagine something similar could be done for behavior modification. An easy built-in scripting language, or perhaps some sort of drag and drop system for creating new behaviors. Anyway, that's what I'd like to see for the next update.

Wah, I haven't played any Team Fortress games before. I'm not sure if I can make it in time.

Luckily, TF2 is free to play, so if you want to give it a try it won't cost you anything. Plus, you don't need a very powerful machine to run the game -- I've played it a number of times on some pretty old hardware!